Image sensor and method of manufacturing the same

ABSTRACT

On a transparent electrically insulating substrate, formed are a scanning line, and a gate electrode of a switching element, further formed are a gate insulating film, a semiconductor layer, an n + -Si layer to be formed into a source electrode and a drain electrode. After the patterning of the foregoing structure, the dielectric film is formed, and the portion corresponding to the contact hole is removed by etching, and photosensitive resin is applied to form the interlayer insulating film. Then, the transparent electrode is extended from the pixel electrode over the switching element, whereon a conversion layer and a gold layer for use in electrode are vapor-deposited. In this structure, an increase in capacitor between the pixel electrode and the signal line can be suppressed by the interlayer insulating film, and the transparent electrode functions as a top gate and release excessive electric charge. As a result, excessive electric charge can be released effectively in the double gate structure while suppressing an increase in capacitor between the pixel electrode and the signal line.

This is a division of application Ser. No. 09/796,676 filed Mar. 2,2001, now U.S. Pat. No. 6,642,541.

FIELD OF THE INVENTION

The present invention generally relates to an image sensor forconverting an incident electromagnetic wave such as a light beam or anX-ray into electric charge and outputting an image signal bysequentially reading out the electric charge, and also relates to amethod of manufacturing such image sensor.

BACKGROUND OF THE INVENTION

A known active matrix substrate for use in a liquid crystal displaydevice, etc., includes a plurality of independently driven pixelelectrodes arranged in a matrix form, and switching elements such asTFTs (Thin Film Transistors), etc., provided for respective pixelelectrodes. In the liquid crystal display device adopting such activematrix substrate, an image is displayed by sequentially selecting theswitching elements by scanning lines and reading potentials of signallines into the pixel electrodes via the switching elements.

The foregoing active matrix substrate can be used for an image sensor.Examples of known image sensors adopting the active matrix substrateinclude: an image sensor including a conversion layer formed on an upperlayer of the active matrix substrate, for directly converting incidentelectromagnetic wave such as a light beam, an X-ray, etc., into electriccharge, wherein the electric charge generated from the conversion layeris stored in pixel capacitance at high voltage, and the electric chargeis read out sequentially from the pixel capacitance. For example,Japanese Unexamined Patent Publication No. 212458/1992 (Tokukaihei4-212458) published on Aug. 4, 1992, discloses an image sensor of theabove type wherein electric charge as generated by the conversion layeris stored in auxiliary capacitance, and data (potential data) are storedin respective pixels in the form of electric charge according to thecharacteristics of an object. As in the case of the aforementionedliquid crystal display device, by sequentially scanning the scanninglines, for example, the data stored in a pixel selected by a scanningline is read out and transmitted via a switching element to a signalline, and an image projected to the image sensor is read out from acircuit such as an operation amplifier provided on the other end of thesignal line.

The active matrix substrate, which is a precursor to the sensor in theforegoing example can be manufactured at low costs without requiring anyadditional facilities, because the manufacturing process for liquidcrystal display devices can be used for the manufacturing process ofimage sensors only by adjusting the dimensions of the pixel capacitanceand the time constants of the switching elements to be optimal for imagesensors.

FIG. 6 is a cross-sectional view illustrating a schematic structure of aknown example of the basic image sensor adopting an active matrixsubstrate. The structure illustrated in FIG. 6 is disclosed in AM-LCD'99“Real-time Imaging Flat Panel X-Ray Detector” by M. Ikeda, et al. Asillustrated in FIG. 6, the active matrix substrate of this sensor isprepared by forming a switching element 51 on a transparent insulatingsubstrate 55, and further vapor-depositing thereon a conversion layer 66and a metal layer 67 in this order. The switching element 51 is preparedby forming on the transparent insulating substrate 55, a gate electrode56, an auxiliary capacitance electrode (not shown), a gate insulatingfilm 57, a semiconductor layer 58, an n⁺—Si layer 59 to be patternedinto a drain electrode, a metal layer 60 and a transparent electricallyconductive film 61 to be patterned into a source signal line, and aprotective film 62 in this order, thereby forming a substrate of theimage sensor. The conversion layer 66 is provided for converting anX-ray into electric charge. The metal layer 67 is patterned into anelectrode for use in applying a voltage to the conversion layer 66. Inthe foregoing structure, the transparent electrically conductive film 61is patterned into the pixel electrodes for storing the electric chargeas converted in the conversion layer 66.

In the image sensor, the electric charge is read out from respectivepixel electrodes in contrast to the liquid crystal display device inwhich electric charge is applied to the pixel electrodes. Therefore, ifa normal readout operation of a predetermined cycle is not performed dueto any failure, or a trouble in signal readout program, unexpectedlylarge electric charge may be stored in the pixel electrode, and theresulting high voltage may cause a damage on the active matrixsubstrate. The foregoing problem is discussed in “Characteristics ofdual-gate thin film transistors for applications in digital radiology”(NRC'96) in “Can. I. Phys. (Suppl)74 published in 1996, in which thefollowing structure has been proposed as a solution to the problem. Thatis, a pixel electrode is extended over a switching element, so that thepixel electrode can be functioned as one of the gate electrodes of adual-gate transistor, and at or above a predetermined threshold voltage,the transistor is switched on, and excessive electric charge isreleased.

The structure of an image sensor which is particularly effective inpreventing the foregoing problem will be explained in reference to FIG.7. As illustrated in FIG. 7, the image sensor has a so-called “mushroomstructure” wherein pixel electrodes 72 and source lines 71 are formed indifferent layers so as to be insulated by an insulating layer 73 formedin-between, so that the entire channel region W of a transistor 74 iscovered with the corresponding pixel electrode 72. In FIG. 7, thereference numerals 75, 76, 77, 78 and 79 indicate a gate electrode, adrain electrode, an auxiliary capacitance, a conversion layer and asemiconductor layer respectively.

The foregoing structure of Waechter, et al, illustrated in FIG. 7 iseffective for the high voltage protection in the pixel electrodes 72. Asto the size of the pixel electrodes 72, however, significant improvementfrom the aforementioned active matrix substrate illustrated in FIG. 6cannot be expected. It is generally known that the larger is the areaoccupied by the pixel electrodes 72, the more efficiently, the electriccharge generated from the conversion layer 78 can be collected in thepixel electrodes 72. In the generally used active matrix substrate,however, there is a limit for an increase in size of each pixelelectrode as pixel electrodes are arranged in a plane with certainintervals from source bus lines.

In the foregoing structure of FIG. 7 wherein the insulating film 73 isformed between the source line 71 and the pixel electrodes 72, the pixelelectrodes 72 can be formed over the source lines 71 while maintainingthe insulation between them. In this state, the electrostaticcapacitance is generated between the pixel electrodes 72 and the sourcelines 71, and an overall capacitance of the source lines 71 when seenfrom the side of the signal readout circuit increases, and a noise ofthe readout signal is increased, resulting in lower signal to noise(S/N) ratio. For the foregoing reasons, the structure of FIG. 7 wouldnot offer any significant improvement in size of the pixel electrodes 72from the conventional active matrix substrate.

In the X-ray image sensor, generally a large pixel capacitance isensured. For this reason, the capacitance between the pixel electrode 72and the source line 71 becomes a load capacitance to the source line 71directly. On the other hand, internal noise generated in the signalreadout amplifier is amplified by a gain in proportion to the ratio ofthe capacitance of the source line 71 to the feedback capacitance. It istherefore effective to reduce the capacitance of the source line 71 fora reduction in internal noise.

Further, an increase in capacitance of the source line 71 may causevariations in potential of the source line 71 corresponding to thecapacitance CsD (per pixel) between the pixel electrode 72 and thesource line 71 with changes in pixel potential corresponding to the partof the image irradiated with an X-ray. For example, when reading outsignals via the source line 71 with a selection of certain scanningline, the electric charge is kept being stored in other pixelelectrodes, while the electric charge in positive polarity and inproportion to the capacitance CsD are being stored in the source line71. The amount of the electric charge to be stored in the pixelelectrodes and the source line 71 differ depending on the image on anentire screen, thereby presenting a problem that a so-called crosstalkis generated when reading out signals as being affected by pixelelectrodes aligned in direction parallel to the source line 71.

In order to reduce the capacitance of the source line 71, for example,an image sensor adopting an interlayer insulating film made ofphotosensitive resin has been proposed, for example, in “Similaritiesbetween TFT Arrays for Direct-Conversion X-Ray Sensors and High-ApertureAMLCDs” (SID 98 DIGEST) by W.den Boer, et al, published in 1998.

However, W.den Boer, et al does not refer to the dual-gate structure.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an image sensor of adual-gate structure which permits excessive electric charge to bereleased effectively while suppressing an increase in capacitancebetween a pixel electrode and a signal line.

In order to achieve the above object, an image sensor of the presentinvention is characterized by including:

a conversion section for converting an incident electromagnetic waveinto electric charge;

pixel electrodes for storing the electric charge generated by theconversion section;

switching elements for controlling reading out of the electric chargefrom the pixel electrodes;

an interlayer insulating film made of an organic film formed under eachpixel electrode;

an electrically conductive film which is electrically connected to thepixel electrodes, and which is extended from the pixel electrode to alayer above each switching element; and

a dielectric layer formed between the switching element and theelectrically conductive film.

According to the foregoing structure, an interlayer insulating film isformed between the scanning lines, signal lines, and pixel electrodes inthe active matrix substrate. It is therefore possible to form the pixelelectrodes over the signal lines. As a result, an improved apertureratio can be achieved, and in the meantime, by shielding the conversionlayer from the electric field generated by the signal lines and thescanning lines, an operation inferior of the conversion layer becomesless likely to occur.

Moreover, the organic film of low dielectric constant can be formedthick with ease, and therefore the electrostatic capacitance between thepixel electrode and the source signal line can be suppressed to besmall. As a result, an increase in noise due to an increase incapacitance of the source signal lines can be prevented, and an improvedsignal to noise (S/N) ratio can be achieved. Furthermore, the activematrix substrate of the image sensor can be manufactured by themanufacturing process of conventional liquid crystal display devicewithout significant modification, and therefore any additional facilityis not needed.

Furthermore, the electrically conductive film is extended from the pixelelectrode over the switching element. Therefore, even if a normalreadout operation of a predetermined cycle is not performed due to anyfailure, or trouble in signal readout program, and unexpectedly largeelectric charge is stored in the pixel electrode, the switching elementis switched ON at or above a predetermined threshold voltage to releasethe excessive electric charge, thereby preventing the switching elementfrom being damaged.

Moreover, by forming the dielectric layer between the switching elementand the electrically conductive film, such characteristic that the thinfilm transistor is switched ON at or above a predetermined thresholdvoltage is determined by the thickness and the dielectric constant ofthe dielectric layer formed between the electrically conductive film andthe switching element, and therefore, the foregoing characteristic canbe set independently of the interlayer insulating film. Namely, whilemaintaining optimal excessive voltage discharge characteristic, theelectrostatic capacitance between the pixel electrode and the sourcesignal line can be suppressed, and in the meantime, the signal to noise(S/N) ratio can be improved.

In the foregoing structure, it may be arranged such that in an areaabove each switching element, the electrically conductive film contactsthe dielectric layer without having the interlayer insulating film inbetween.

In the foregoing structure, it may be arranged such that the switchingelement is a dual-gate transistor, and the electrically conductive filmfunctions as one of gate electrodes of the dual-gate transistor.

In the foregoing structure, it may be arranged such that each switchingelement including its channel region is covered with the dielectriclayer,

the electrically conductive film is extended from the pixel electrode toan area above the switching element including its channel region, and

in an area above each switching element, the electrically conductivefilm contacts the dielectric layer without having the interlayerinsulating film in between.

In the foregoing structure, it may be arranged such that the electriccharge stored in the pixel electrode is a positive charge, and theswitching element conducts with an application of a positive biasvoltage.

Alternatively, it may be arranged such that the electric charge storedin the pixel electrodes is negative charge, and the switching elementconducts with an application of a negative bias voltage.

In the foregoing structure, it may be arranged such that in an areaabove the switching element, the interlayer insulating film is formedbetween the dielectric layer and the electrically conductive film.

According to the foregoing structure, in the area above the switchingelement, which has the roughest surface in the active matrix substratesformed are not only the dielectric layer but also the interlayerinsulating film made of an organic film. With this structure, even suchprotrusions and recessions of the rough surface, which cannot beabsorbed completely by the dielectric layer alone, can be absorbed to asufficient level. In this structure, even when adopting a conversionlayer made of selenium, it is still possible to suppress crystallizationdue to the protrusions and recessions, and therefore films can be formedunder stable conditions.

In the foregoing structure, the interlayer insulating film may bestructured such that at least a portion above the switching element isformed thinner than other portion of the interlayer insulating film.

In the foregoing structure, the excessive voltage dischargecharacteristic is determined by the thickness and the dielectricconstant of the interlayer insulating film in the portion between theelectrically conductive film extended from the pixel electrode and theswitching element. It is therefore possible to set the foregoingcharacteristic independently of the interlayer insulating film in theportion for use in forming the electrostatic capacitance between thepixel electrode and the source electrode. Namely, with the foregoingstructure, an improved S/N ratio can be achieved while maintainingoptimal excessive voltage discharge characteristic.

In the foregoing structure, the interlayer insulating film may bestructured such that at least a portion corresponding to the channelregion of the switching element is formed thinner than other portion ofthe interlayer insulating film.

In the foregoing structure, a photosensitive organic film may be adoptedas the interlayer insulating film.

According to the foregoing structure, in the area above the switchingelement, which has the roughest surface in the active matrix substrate,formed are not only the dielectric layer but also the interlayerinsulating film made of the organic film. With this structure, even suchprotrusions and recessions of the rough surface, which cannot beabsorbed completely by the dielectric layer alone, can be absorbed to asufficient level. In this structure, even when adopting a conversionlayer made of selenium, it is still possible to suppress crystallizationdue to the protrusions and recessions, and therefore films can be formedunder stable conditions.

In order to achieve the foregoing object, another image sensor forconverting incident electromagnetic wave into electric charge by each ofa plurality of pixel electrodes and outputting image signals bysequentially reading out the electric charge from the pixel electrodesvia switching elements, is characterized by including:

an electrically conductive film formed so as to be extended from thepixel electrode to a portion above each switching element; and

an interlayer insulating film made of an organic film, formed below eachpixel electrode and the electrically conductive film, the interlayerinsulating film being structured such that a portion above the switchingelement is thinner than other portion of the interlayer insulating film.

In the foregoing structure, the excessive voltage dischargecharacteristic is determined by the thickness of the dielectric constantof the portion between the electrically conductive film extended fromthe pixel electrode and the switching element, and therefore, it ispossible to set the foregoing characteristic independently of theinterlayer insulating film in the portion for use in forming theelectrostatic capacitance between the pixel electrode and the sourceelectrode. Namely, with the foregoing structure, an improved S/N ratiocan be achieved while maintaining optimal excessive voltage dischargecharacteristic.

In the foregoing structure, it may be arranged such that the interlayerinsulating film is structured such that at least a portion correspondingto the channel region of the switching element is formed thinner thanother portion of the interlayer insulating film.

In the foregoing structure, it may be arranged such that the channelregion of the switching element contacts the interlayer insulating film.

In the foregoing structure, an inorganic film may be adopted for thedielectric layer.

In the foregoing structure, it may be arranged such that:

a double layer structure of the dielectric film of an inorganic film andthe interlayer insulating film of an organic film is formed under thepixel electrode,

in an area above each switching element, the electrically conductivefilm contacts the dielectric layer without having the interlayerinsulating film in between.

In the foregoing structure, it may be arranged so as to further include:

a signal line for transferring charge as collected in each pixelelectrode via a switching element,

wherein the pixel electrode is formed over the signal line having theinterlayer insulating film in between.

In order to achieve the above object, a method of manufacturing an imagesensor of the present invention is characterized by including the stepsof:

forming a plurality of switching elements, a plurality of scanning linesand a plurality of signal lines on an insulating substrate;

forming an interlayer insulating film made of a photosensitive organicfilm in respective portions above the plurality of switching elements,scanning lines and signal lines,

exposing and developing a resulting photosensitive organic film;

forming pixel electrodes on the interlayer insulating film; and

forming conversion means on the pixel electrodes for converting anincident electromagnetic wave into electric charge,

wherein exposure with respect to the photosensitive organic film isvaried between at least a portion of an area above each switchingelement and other portion of the photosensitive organic film.

According to the foregoing structure, the protrusions and recessionsresulting from the patterning of the wires in layer below the interlayerinsulating film can be suppressed by the interlayer insulating film, andan inferior in characteristics of the conversion means for converting anincident X-ray into electric change in upper layer can be prevented.Moreover, by adopting photosensitive resin, a smooth cross section canbe achieved even at the pattern edge of the interlayer insulating film,and therefore it is possible to more surely prevent an inferiorcharacteristic of the conversion means. Furthermore, as the pixelelectrodes can be formed over the source electrodes, an area occupied bythe pixel electrodes can be increased, and therefore, it is possible tocollect the electric charge generated from the conversion means in anefficient manner. In the foregoing structure, even if a normal readoutoperation of a predetermined cycle is not performed due to any failure,or trouble in signal readout program, and unexpectedly large electriccharge is stored in the pixel electrode, the switching element isswitched ON at or above a predetermined threshold voltage to release theexcessive electric charge, thereby preventing the switching element frombeing damaged. Moreover, while maintaining optimal excessive voltagedischarge characteristics, the electrostatic capacitance between thepixel electrode and the source signal line can be suppressed, and in themeantime, the signal to noise (S/N) ratio can be improved.

For a fuller understanding of the nature and advantages of theinvention, reference should be made to the ensuing detailed descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view illustrating an image sensor in accordance withone embodiment of the present invention.

FIG. 2 illustrates a cross section of the portion along an arrow lineB—B of the image sensor of FIG. 1.

FIG. 3 is a cross sectional view of an image sensor in accordance withanother embodiment of the present invention.

FIG. 4 is a cross sectional view of an image sensor in accordance withstill another embodiment of the present invention.

FIG. 5 is a cross sectional view of a modified example of the imagesensor of FIG. 4 from which a dielectric layer is omitted.

FIG. 6 is a cross sectional view of a conventional image sensor adoptingan active matrix substrate.

FIG. 7 is a cross sectional view illustrating the schematic structure ofanother conventional image sensor.

FIG. 8 is a cross sectional view illustrating the schematic structure ofan image sensor.

FIG. 9 is a cross sectional view illustrating the schematic structure ofanother image sensor.

DESCRIPTION OF THE EMBODIMENTS

To begin with, an image sensor in which pixel electrodes can be formedover source signal lines will be explained.

The structure of the foregoing image sensor is illustrated in FIG. 8. InFIG. 8, members having the same functions as those illustrated in FIG. 6will be given the same reference symbols.

In this image sensor, when forming a protective film 62 on a transparentconductive film 61, the portion corresponding to a contact hole 65 ofthe protective film 62 is removed by etching, and then photosensitiveresin is applied by the spin coating method to form an interlayerinsulating film 63. Thereafter, a contact hole 65 is formed by thenormal photographic process, and a pixel electrode 64 formed on theinterlayer insulating film 63 is connected to the drain electrode of theswitching element 51 via the contact hole 65.

In the active matrix substrate manufactured in the foregoing method,wherein the interlayer insulating film 63 is formed between the scanninglines and signal lines, it is therefore possible to form the pixelelectrode 64 over the signal line. With the foregoing structure, animproved aperture ratio can be achieved. Further, by shielding theconversion layer 66 from the electric field generated by the signallines and the scanning lines, an operation inferior of the conversionlayer 66 can be suppressed.

Although the pixel electrode 64 is formed over the source signal line,by adopting the interlayer insulating film 63 of a sufficient thicknesswith low dielectric constant, an increase in capacitance and a reductionin S/N can be prevented. Moreover, a slighter greater amount of signalscollected that of the image sensor adopting the active matrix substrateillustrated in FIG. 6.

Furthermore, by adopting the dual-gate structure in the foregoingstructure of FIG. 8, an image sensor illustrated in FIG. 9 can beachieved. In FIG. 9, members having the same functions as thoseillustrated in FIG. 8 will be given the same reference symbols. In theimage sensor of FIG. 9, a transparent electrically conductive film 64 aextended from the pixel electrode 64 is formed via the interlayerinsulating film 63. Here, the structure in which the protective film 62is removed in an area above the switching element 51 will be considered.

As described, when a normal operation is not performed in an event of atrouble or an error in signal readout program, electric charge inpositive polarity is kept being applied onto the pixel electrode 64 asillustrated in FIG. 9. In the previous structure of FIG. 8, a voltagemay be increased to the level at which the switching element 51 isdestroyed. In contrast, according to the structure illustrated in FIG. 9adopting the double gate structure, when a pixel potential is raised toa predetermined threshold voltage, the extended transparent electricallyconductive film 64 a functions as one of the dual-gate electrodes, andthe switching element 51 conducts at low conductivity, and excessiveelectric charge is released to the source signal line.

In the conventional excessive electric charge protection structure suchas the mushroom structure illustrated in FIG. 7, silicon nitride orsilicon oxide is deposited between the top gate 72 and the semiconductorlayer 79 as in the typical active matrix substrate. As silicon nitrideor silicon oxide has a high dielectric constant, due to restrictions inprocessing or film formation time, it is not possible to deposit suchfilm made of silicon nitride or silicon oxide as thick as a film made ofresin formed by the spin coating method. Therefore, in the resultingstructure, it is likely that electric charges is released at relativelylow potential. It is therefore not possible to store a large amount ofelectric charge resulting in lower S/N. In view of a storage amount ofelectric charge, the aforementioned structure of FIG. 9 is advantageousover the foregoing structure of FIG. 7. As is clear from the foregoingexplanations, the interlayer insulating film 63 desirably has lowdielectric constant and is thick. However, when adopting such interlayerinsulating film 63, a problem arises in that the conductivity of theswitching element 51 by the top gate of the dual-gate becomes too low.Namely, the structure of FIG. 9 adopting the dual-gate structure may beeffective for increasing the capacitance between the pixel electrode 64and the source signal line depending on the property of the interlayerinsulating film adopted, however, the function of releasing theexcessive electric charge which is the advantageous characteristic ofthe dual-gate structure cannot be fully appreciated.

Depending on the amount of electric charge generated from the conversionlayer 66 and the pixel capacitance, only a small amount of electriccharge may be stored in the pixel electrodes 64. However, in this case,even if the electric charge starts being released by the top gate beforethe voltage is raised above a threshold high voltage, this would not bea problem. In the case of dealing with very small amount of electriccharge like the case of the image sensor, variations among pixels of theswitching elements 51 cause a serial problem, and therefore it is ratherpreferable to adopt such structure that the electric charge is releasedat potential slightly higher than the threshold potential.

Namely, when an unexpectedly high voltage of around several tens V isapplied to the switching element 51 which is unlikely to generate in thenormal operation, even once, a trap potential may be formed in the gateinsulating film 57 or abnormality in characteristic occurs due to theelectric charge being captured. The foregoing abnormality incharacteristic possibly occurs even at relatively low voltage. Forexample, when for some reason, a scanning is stopped in the middle ofthe operation while maintaining signal charge in the pixels, suchabnormality in characteristic possibly occurs even at low voltage withan application of a DC voltage over a long time. Especially, for theimage sensor which deals with signals of very low level, deviations inthe characteristic from the normal value would directly cause deviationsin amount of charge of the signal to be collected. Therefore, thestructure for preventing abnormality in characteristic is needed.

However, in the foregoing structure of FIG. 9, the switching element 51conducts only at voltage of sufficiently high level, and the electriccharge cannot be released at voltage slightly higher than the thresholdvoltage.

In view of the foregoing, an image sensor of a dual-gate structure whichpermits excessive electric charge to be released effectively whilesuppressing an increase in capacitance between a pixel electrode and asignal line will be explained in the following first embodiment of thepresent invention.

First Embodiment

The following descriptions will discuss one embodiment of the presentinvention in reference to FIG. 1 and FIG. 2.

FIG. 1 is a front view of an image sensor in accordance with oneembodiment of the present invention. FIG. 2 is a cross section of theportion along an arrow line B—B in FIG. 1. The image sensor of thepresent embodiment includes a transparent insulating substrate 5, havingformed thereon a scanning line 3, and a gate electrode 6 and anauxiliary capacitance line of a switching element 1. Further laminatedthereon are a gate insulating film 7, a semiconductor layer 8, and ann⁺—Si layer 9 to be patterned into a source electrode and a drainelectrode. Then, the foregoing laminated structure is subjected topatterning. On the n⁺—Si layer 9, further laminated are a transparentconductive layer 11 and a metal layer 10 for use in a source signal line4, and the resulting laminated structure is subjected to the patterning.

The wires and the patterns have a double layer structure as in theforegoing, for purpose of purpose of suppressing problem associated witha disconnection of a wire due to dust particles accumulated on a layerand preventing the base layer from being damaged during the patterningof the overlying metal layer.

Then, a dielectric layer 22 is formed, and the portion corresponding tothe contract hole 15 is removed by etching. Thereafter, photosensitiveacrylic transparent resin is applied by the spin coating method, therebyforming an inter-layer insulating film 13. For this resin, for example,positive type photosensitive resin may be adopted. The resin has adielectric constant of 3, and a thickness of 2 μm. Then, as in thenormal photographic process, the developing is performed by exposing thearea above the switching element 1 and the portion corresponding to thecontact hole 15. As described, after forming an interlayer insulatingfilm 13, a transparent conductive layer to be patterned into pixelelectrodes 14 is formed, and is subjected to the patterning by etching.Here, the pixel electrode 14 is connected to the drain electrode of theswitching element 1 via the contact hole 15 through the dielectric layer22 and the interlayer insulating film 13.

The essential feature of the image sensor of the present embodiment liesin that in an area above the switching element 1, the transparentdielectric layer 14 b extended from the pixel electrode 14 is formed viathe dielectric layer 22. The dielectric layer 22 is adopted also in theconventional active matrix substrate as a protective film for thepurpose of improving the reliability of the switching element 1. Forthis dielectric layer 22, silicon nitride or silicon oxide is typicallyused. The dielectric film 22 is formed in thickness of several thousandsÅ. In the area above the switching element 1, the interlayer insulatingfilm 13 is removed by exposure and development, wherein the transparentdielectric layer 14 b extended from the pixel electrode 14 contacts theprotective film 22.

In the image sensor of the present embodiment, a double layer structureof the dielectric film 22 of an inorganic film and the interlayerinsulating film 13 of an organic film is formed under the pixelelectrode 14, and the interlayer insulating film 13 is eliminated onlyfrom the area above the switching element 1.

On the upper layer of the foregoing active matrix substrate, aconversion layer 16 made of selenium is vapor-deposited, for example, bythe vacuum deposition. On the conversion layer 16, deposited is a goldlayer 17 to be patterned into an electrode for applying a voltage to theconversion layer 16, thereby forming a substrate of the image sensor. Inthis example, the conversion layer, a positive bias voltage is appliedby a power source 18.

According to the foregoing active matrix substrate, the interlayerinsulating film 13 is formed between the scanning lines 3, the scanninglines 4, and the pixel electrodes 14, and it is therefore possible toform the pixel electrodes 14 over the signal lines 4. With the forgoingcharacteristic structure, the aperture ratio can be improved, and at thesame time, operation inferior of the conversion layer 16 can besuppressed by shielding it from the electric field generated by thesignal lines 4 and the scanning lines 3. By adopting the interlayerinsulating film 13 of a sufficient thickness and of low dielectricconstant, an increase in capacitance and a reduction in S/N ratio can besuppressed even in the foregoing structure of forming the pixelelectrodes 14 over the source signal lines 4.

In the area above the switching element 1, the transparent conductivelayer 14 b extended from the pixel electrode 14 is formed via thedielectric layer 22. With this structure, the transparent conductivelayer 14 a functions as a top gate of the dual-gate transistor, and theswitching element 1 conducts at potential slightly higher than thethreshold potential, thereby releasing excessive electric charge intothe source signal line 4.

In the foregoing structure, when the electric change is released at apotential slightly higher than the threshold potential, the DC voltageof low level is applied to the switching element 1, and when a voltageof higher level is applied, the switching element 1 strongly conductsand can release the electric charge at extremely low time constant.Namely, as compared to the interlayer insulating film 63 of several μm,the dielectric layer 22 in the thickness of several hundreds of Å wherethe pixel electrode is closer to the gate insulating film 7 for use inthe gate electrode 6 and the bottom gate of the dual gate is more suitedfor the above condition, and thus offers greater effects of preventingthe foregoing problem of variations in characteristics among pixels.

The dielectric layer 22 is formed by the vapor deposition such as CVD(Chemical Vapor Deposition) method whose thickness can be adjusted in asimpler manner than the film thickness adjustment of the interlayerinsulating film 63 (see FIG. 8) formed by the spin coating method.Moreover, the dielectric layer 22 made of silicon nitride or siliconoxide offers stable properties unlike the organic film. Therefore,changes in characteristics of the insulating film caused by the electricfield in the operation of the optical sensor is less likely to occur.The foregoing advantageous characteristics are very important inpreventing an increase in leak current due to variations in switchingeffects by the top gate being used or deteriorations in high voltageprotection function.

In the foregoing preferred embodiment, explanations have been giventhrough the case wherein positive electric charge is stored in the pixelelectrode 14, and when excessive electric charge is stored in the pixelelectrode 14, a positive bias voltage is applied to the switchingelement 1 to conduct it, to allow excessive electric charge to bereleased. However, the switching element of the present invention is notlimited to the above, and for example, p-type channel transistor may beadopted. In this case, negative electric charge generated from theconversion layer 16 with an application of negative vias charge isstored in the pixel electrode 14, and when excessive electric charge isstored in the pixel electrode 14, negative bias voltage is applied tothe switching element 1 to conduct it to allow excessive electric chargeto be released. Needless to mention, the foregoing structure using anegative bias voltage offers the same effects as achieved from theforegoing structure of the above preferred embodiment.

Second Embodiment

The following descriptions will discuss another embodiment of thepresent invention in reference to FIG. 3. For ease of explanation,members (structures) having the same functions as those shown in thedrawings pertaining to the first embodiment above will be given the samereference symbols, and explanation thereof will be omitted here.

FIG. 3 is a cross sectional view of an image sensor in accordance withthe second embodiment of the present invention. The image sensor of thepresent embodiment basically has the same structure as the image sensorof the first embodiment illustrated in FIG. 1 and FIG. 2 except for thefollowing structure. In the image sensor of the present embodiment, inan area above the switching element 1, a transparent conductive layer 14a extended from the pixel electrode 14 is formed via not only aprotective film 12 (dielectric layer) but also the interlayer insulatingfilm 13. Namely, the image sensor of the present embodiment differs fromthe image sensor of the previous embodiment in that, the interlayerinsulating film 13 and the transparent electrically conductive film 14 aare formed on the protective film 12 in the channel region of theswitching element 1.

The foregoing structure of the present embodiment offers the effect asachieved from the structure illustrated in FIG. 9 and the effect asachieved from the structure illustrated in FIG. 1 and FIG. 2, i.e., animproved reliability of the switching element 1, and prevention of aleak current due to a top gate at low voltage. Moreover, according tothe structures illustrated in FIG. 1 and FIG. 2, in the area above theswitching element, which has the roughest surface in the active matrixsubstrate contacts the conversion layer 16 only via the dielectric layer22, and with the single use of this dielectric layer 22, the roughnessmay not be absorbed completely. In response, the image sensor of thepresent embodiment is structured so as to form the interlayer insulatingfilm 13 and the transparent conductive layer 14 a on the protective film12 (dielectric layer) to make the surface smoother. With this structure,even when adopting the conversion layer 16 made of selenium which isliable to be crystallized by the protrusions and recessions of the roughsurface, it is still possible to form films under stable conditions.

The thickness of the interlayer insulating film 13 may be selected suchthat optimal characteristics can be achieved when used in combinationwith the protective film 12. Specifically, first, the total amount ofelectrostatic capacitance is set such that i) the current-voltagecondition in which the switching element 1 can be surely prevented frombeing destroyed against high voltage, and ii) the condition ofpreventing leak current in normal operations can be well-balanced, andthe film thickness of the protective film 12 (inorganic film) and thefilm thickness of interlayer insulating film 13 (organic film) whichprevents an inferior in the conversion layer 16 may be set so as tosatisfy the total amount of electrostatic capacitance as set.

Third Embodiment

The following descriptions will discuss still another embodiment of thepresent invention in reference to FIGS. 4 and 5. For ease ofexplanation, members (structures) having the same functions as thoseshown in the drawings pertaining to the first and second embodimentsabove will be given the same reference symbols, and explanation thereofwill be omitted here.

FIG. 4 shows a cross sectional view of an image sensor in accordancewith the third embodiment of the present invention. The image sensor ofthe present embodiment differs from the image sensor of the secondembodiment illustrated in FIG. 3 in the following structure. That is,the interlayer insulating film 13 is constituted by an interlayerinsulating film 13 a formed in an area above the switching element 1,and an interlayer insulating film 13 b of the remaining portion. Theinterlayer insulating film 13 is structured such that the interlayerinsulating film 13 a and the interlayer insulating film 13 b havedifferent thicknesses. Specifically, the thickness of the interlayerinsulating film 13 a is set such that the switching element 1 can beprevented from being destroyed against high voltage and in the meantimeleak current in the normal operation can be prevented. On the otherhand, the interlayer insulating film 13 b of the remaining portion isformed thicker than the interlayer insulating film 13 a, i.e., 2 μm sothat a level difference at a portion where the source signal line 4 anda scanning signal line 3 crosses, and a reduction in capacitance betweenthe pixel electrode 14 and the source signal line 4 can be surelysuppressed.

By adopting the photosensitive organic film for the interlayerinsulating film 13, the foregoing structure can be realized with ease bythe following method. Firstly, the process up to the formation of themetal layer 10 and the transparent electrically conductive film 11 areperformed by the known method of manufacturing a active matrixsubstrate. Then, the dielectric layer (protective film) 12 is formed,and the portion corresponding to the contact hole 15 of the dielectriclayer 12 is removed by etching. Then, photosensitive acrylic transparentresin is applied to the thickness of 2 μm by the spin coating method.Further, after exposing on the switching element 1 with an ultravioletray of low intensity, or with an ultraviolet ray of normal intensity,the portion corresponding to the contact hole 15 is fully exposed. Asthe acrylic transparent resin is positive photosensitive resin, theexposed portion can be removed in the same developing process as that ofthe normal photographic process. On the other hand, the portion abovethe switching element 1 is not fully exposed, and therefore, the upperlayer is not removed be removed completely by developing, and theresidual layer on the switching element 1 is formed in an insulatingthin film 13 a.

In the foregoing manner, the active matrix substrate of the image sensorcan be formed with ease only by switching exposure without increasingthe number of steps in the manufacturing process. Specifically, afterforming the interlayer insulating films 13 a and 13 b of differentthicknesses, the transparent insulating layer for use in forming thepixel electrode 14 is formed, and is patterned by etching, therebyforming the active matrix part of the image sensor. With the forgoingmethod, both a) the portion from which the interlayer insulating film 13is removed completely, and b) the portion where the thin film portion 13a is formed thin have smooth cross sections, as in the characteristic ofthe photosensitive organic layer formed by developing in thephotographic process, and therefore, the likelihood of the foregoingproblem can be prevented.

According to the structure illustrated in FIG. 4, the double layerstructure of the protective film 12 (dielectric layer) and theinterlayer insulating film 13 are formed between the switching element 1and the pixel electrode 14. However, according to the structure ofadjusting the film thickness of the interlayer insulating film 13 a onthe switching element 1, the discharge characteristic of an excessivevoltage can be controlled by adjusting the film thickness of theinterlayer insulating film 13. It is therefore possible to eliminate thedielectric film 22 provided that the reliability of the switchingelement 1 can be ensured. Without the dielectric layer 22, asillustrated in FIG. 5, the semiconductor layer 8 in the channel regionof the switching element 1 is in direct contact with the interlayerinsulating film 13 a.

In the case where the organic film contacts the channel portion, due todispersion of impurities from the interlayer insulating film 13 a of anorganic film into the semiconductor layer 8 in the channel region, or atrap level on the interface between the interlayer insulating film 13 aand the semiconductor layer 8, it is possible that the abnormality incharacteristics of the switching element 1 occurs, or the reliabilily ofthe switching element 1 may not be ensured. In that case, the structureof FIG. 4 should be adopted. On the other hand, in the case thedesirable characteristic and reliability of the switching element 1 canbe ensured, the structure as illustrated in FIG. 5 without thedielectric layer 22 may be adopted.

In the foregoing first through third preferred embodiments, the activematrix substrate, which is a precursor to the sensor in the foregoingexample can be manufactured at low costs without requiring anyadditional facilities, because the manufacturing process for liquidcrystal display devices can be used for the manufacturing process ofimage sensors only by adjusting the dimensions of the pixel capacitanceand the time constants of the switching elements to be optimal for imagesensors.

As described, an image sensor of the present invention for convertingincident electromagnetic wave into electric charge by each of aplurality of pixel electrodes and sequentially reading the electriccharge from the pixel electrodes via switching elements, so as to outputimage signals, is characterized by including:

an electrically conductive film formed so as to be extended from thepixel electrode to a layer above the switching element; and

an interlayer insulating film made of an organic film, formed in a layerbelow each pixel electrode and the electrically conductive film, theinterlayer insulating film being structured such that a portion abovethe switching element is thinner than the rest of the interlayerinsulating film.

According to the foregoing structure, an interlayer insulating film isformed between the scanning lines, signal lines, and pixel electrodes inthe active matrix substrate. It is therefore possible to form the pixelelectrodes over the signal lines. As a result, an improved apertureratio can be achieved, and in the meantime, by shielding the conversionlayer from the electric field generated by the signal lines and thescanning lines, an operation inferior of the conversion layer becomesless likely to occur.

Moreover, the organic film of low dielectric constant can be formedthick with ease, and therefore the electrostatic capacitance between thepixel electrode and the source signal line can be suppressed to besmall. As a result, an increase in noise due to an increase incapacitance of the source signal lines can be prevented, and an improvedsignal to noise (S/N) ratio can be achieved. Furthermore, the activematrix substrate of the image sensor can be manufactured by themanufacturing process of conventional liquid crystal display devicewithout significant modification, and therefore any additional facilityis not needed.

Furthermore, the electrically conductive film is extended from the pixelelectrode over the switching element. Therefore, even if a normalreadout operation of a predetermined cycle is not performed due to atrouble or an error in signal readout program, and unexpectedly largeelectric charge is stored in the pixel electrode, the switching elementis switched ON at or above a predetermined threshold voltage to releasethe excessive electric charge, thereby preventing the switching elementfrom being damaged.

Moreover, by forming the dielectric layer between the switching elementand the electrically conductive film, such characteristic that the thinfilm transistor is switched ON at or above a predetermined thresholdvoltage is determined by the thickness and the dielectric constant ofthe dielectric layer formed between the electrically conductive film andthe switching element, and therefore, the foregoing characteristic canbe set independently of the interlayer insulating film. Namely, whilemaintaining optimal excessive voltage discharge characteristics, theelectrostatic capacitance between the pixel electrode and the sourcesignal line can be suppressed, and in the meantime, the signal to noise(S/N) ratio can be improved.

The foregoing image sensor of the present invention may be arranged suchthat in an area above the switching element, the interlayer insulatingfilm is formed between the dielectric layer and the electricallyconductive film.

According to the foregoing structure, in the area above the switchingelement, which has the roughest surface in the active matrix substrate,formed are not only the dielectric layer but also the interlayerinsulating film made of the organic film. With this structure, even suchprotrusions and recessions of the rough surface, which cannot beabsorbed completely by the dielectric layer alone, can be absorbed to asufficient level. In this structure, even when adopting a conversionlayer made of selenium, it is still possible to suppress crystallizationdue to the protrusions and recessions, and therefore films can be formedunder stable conditions.

In the image sensor of the foregoing structure, the interlayerinsulating film is structured such that at least a portion correspondingto the channel region of the switching element is formed thinner thanother portion of the interlayer insulating film.

In the foregoing structure, the excessive voltage dischargecharacteristic is determined by the thickness and the dielectricconstant of the interlayer insulating film in the portion between theelectrically conductive film extended from the pixel electrode and theswitching element. It is therefore possible to set the foregoingcharacteristic independently of the interlayer insulating film in theportion for use in forming the electrostatic capacitance between thepixel electrode and the source electrode. Namely, with the foregoingstructure, an improved S/N ratio can be achieved while maintainingoptimal excessive voltage discharge characteristic.

Another image sensor of the present invention for converting incidentelectromagnetic wave into electric charge by each of a plurality ofpixel electrodes and outputting image signals by sequentially readingout the electric charge from the pixel electrodes via switching elementsis characterized by including:

an electrically conductive film formed so as to be extended from thepixel electrode to a portion above each switching element; and

an interlayer insulating film made of an organic film, formed below eachpixel electrode, the interlayer insulating film being structured suchthat a portion above the switching element is thinner than other portionof the interlayer insulating film.

In the foregoing structure, the excessive voltage dischargecharacteristic is determined by the thickness of the dielectric constantof the portion between the electrically conductive film extended fromthe pixel electrode and the switching element, and therefore, it ispossible to set the foregoing characteristic independently of theinterlayer insulating film in the portion for use in forming theelectrostatic capacitance between the pixel electrode and the sourceelectrode. Namely, with the foregoing structure, an improved S/N ratiocan be achieved while maintaining optimal excessive voltage dischargecharacteristic.

The foregoing image sensor of the present invention may be characterizedin that the interlayer insulating film is made of a photosensitiveorganic film.

According to the foregoing structure, in the area above the switchingelement, which has the roughest surface in the active matrix substrate,formed are not only the dielectric layer but also the interlayerinsulating film made of the organic film. With this structure, even suchprotrusions and recessions of the rough surface, which cannot beabsorbed completely by the dielectric layer alone, can be absorbed to asufficient level. In this structure, even when adopting a conversionlayer made of selenium, it is still possible to suppress crystallizationdue to the protrusions and recessions, and therefore films can be formedunder stable conditions.

The method of manufacturing an image sensor of the present invention ischaracterized by including the steps of:

forming a plurality of switching elements, a plurality of scanning linesand a plurality of signal lines on an insulating substrate;

forming an interlayer insulating film made of a photosensitive organicfilm in respective portions above the plurality of switching elements,scanning lines and signal lines,

exposing and developing a resulting photosensitive organic film;

forming pixel electrodes on the interlayer insulating film; and

forming conversion means on the pixel electrodes for converting anincident electromagnetic wave into electric charge,

wherein exposure with respect to the photosensitive organic film isvaried between at least a portion of an area above each switchingelement and other portion of the photosensitive organic film.

According to the foregoing structure, the protrusions and recessionsresulting from the patterning of the wires in layer below the interlayerinsulating film can be suppressed by the interlayer insulating film, andan inferior in characteristics of the conversion means for converting anincident X-ray into electric change on the upper layer can be prevented.Moreover, by adopting photosensitive resin, a smooth cross section canbe achieved even at the pattern edge of the interlayer insulating film,and therefore it is possible to more surely prevent an inferiorcharacteristic of the conversion means. Furthermore, as the pixelelectrodes can be formed over the source electrodes, an area occupied bythe pixel electrodes can be increased, and therefore, it is possible tocollect the electric charge generated from the conversion means in anefficient manner. In the foregoing structure, even if a normal readoutoperation of a predetermined cycle is not performed due to a trouble oran error in signal readout program, and unexpectedly large electriccharge is stored in the pixel electrode, the switching element isswitched ON at or above a predetermined threshold voltage to release theexcessive electric charge, thereby preventing the switching element frombeing damaged. Moreover, while maintaining optimal excessive voltagedischarge characteristics, the electrostatic capacitance between thepixel electrode and the source signal line can be suppressed, and in themeantime, the signal to noise (S/N) ratio can be improved.

Furthermore, the interlayer insulating film of the portion determiningthe excessive voltage discharge characteristic of the switching elementand the interlayer insulating film of the portion determining theelectrostatic capacitance between the pixel electrode and the sourcesignal line can be formed in thickness as desired by adjusting theexposure, thereby controlling respective physical values to be optimalvalues with ease without increasing the manufacturing steps.

Such variations are not to be regarded as a departure from the spiritand scope of the invention, and all such modification as would beobvious to one skilled in the art are intended to be included within thescope of the following claims.

1. An image sensor comprising: a conversion section for converting anincident electromagnetic wave into electric charge; pixel electrodes forstoring the electric charge generated by said conversion section;switching elements for controlling reading out of the electric chargefrom the pixel electrodes; an interlayer insulating film made of anorganic film formed under each pixel electrode; an electricallyconductive film which is electrically connected to said pixelelectrodes, and which is extended from the pixel electrode to a layerabove each switching element; and a dielectric layer formed between theswitching element and said electrically conductive film; wherein in anarea above each switching element, said electrically conductive filmcontacts said dielectric layer without having said interlayer insulatingfilm in between.
 2. An image sensor, comprising: a conversion sectionfor converting an incident electromagnetic wave into electric charge:pixel electrodes for storing the electric charge generated by saidconversion section; switching elements for controlling reading out ofthe electric charge from the pixel electrodes; an interlayer insulatingfilm comprising an organic film formed under each pixel electrode; anelectrically conductive film which is electrically connected to saidpixel electrodes, and which extends from the pixel electrode to a layerabove each switching element; a dielectric layer formed between theswitching element and said electrically conductive film; wherein: saidswitching element is a dual-gate transistor, and said electricallyconductive film functions as one gate electrode of said dual-gatetransistor; each switching element including its channel region iscovered with said dielectric layer, said electrically conductive film isextended from the pixel electrode to an area above said switchingelement including its channel region, and in an area above eachswitching element, said electrically conductive film contacts saiddielectric layer without having said interlayer insulating film inbetween.
 3. An image sensor comprising: a conversion section forconverting an incident electromagnetic wave into electric charge; pixelelectrodes for storing the electric charge generated by said conversionsection; switching elements for controlling reading out of the electriccharge from the pixel electrodes; an interlayer insulating film made ofan organic film formed under each pixel electrode; an electricallyconductive film which is electrically connected to said pixelelectrodes, and which is extended from the pixel electrode to a layerabove each switching element; and a dielectric layer formed between theswitching element and said electrically conductive film; wherein adouble layer structure of the dielectric film of an inorganic film andthe interlayer insulating film of an organic film is formed under saidpixel electrode, and in an area above each switching element, saidelectrically conductive film contacts said dielectric layer withouthaving said interlayer insulating film in between.